The present invention generally relates to rotating electromagnetic components, such as rotors for motors and generators. More particularly, this invention relates to a method using powder technologies to net-shape mold a rotating electromagnetic component having soft and permanent magnet core components.
The use of powder metallurgy (P/M), and particularly iron and iron alloy powders, is known for forming magnets, including soft magnetic cores for transformers, inductors,.AC and DC motors, generators, and relays. An advantage to using powdered metals is that forming operations, such as compression molding, injection molding and sintering techniques, can be used to form intricate molded part configurations, such as magnetic cores, without the need to perform additional machining and piercing operations. As a result, the formed part is often substantially ready for use immediately after the forming operation. For rotating components such as rotors for AC and DC motors and generators, soft magnets can be sintered at temperatures of about 2050xc2x0 F. (about 1120xc2x0 C.) or more to achieve greater mechanical strength.
Certain electromagnetic applications require or benefit from a combination of soft iron magnets and xe2x80x9chardxe2x80x9d permanent magnet, such as when permanent magnet regions are provided within a rotor to direct magnetic flux, yielding a more efficient rotor with higher outputs. A prominent example of a permanent magnet for such applications is based on compositions containing iron, a rare earth metal such as neodymium and/or praseodymium, and boron. Such permanent magnets contain, as an essential magnetic phase, grains of 2 tetragonal crystals in which the proportions of iron, neodymium and boron (for example) are exemplified by the empirical formula Fe14Nd2B. These magnet compositions and methods for making them are described in U.S. Pat. No. 4,802,931 to Croat. The grains of the magnetic phase are surrounded by a second phase that is typically rare earth-rich, e.g., neodymium-rich, as compared to the essential magnetic phase. Magnets based on such compositions are typically prepared by rapidly solidifying (such as by melt spinning) a melt of the composition to produce fine grained, magnetically isotropic platelets of ribbon-like fragments with an amorphous noncrystalline metallurgical structure. High coercivity permanent magnets may be formed from these isotropic particles by blending the fragments with a binder followed by compacting to form a xe2x80x9csoftxe2x80x9d magnetically isotropic magnet body. According to U.S. Pat. No. 4,782,367 to Lee, stronger magnets can be produced by hot pressing the magnet body to gain some magnetic anisotropy in the direction in which the magnet body was hot pressed, and still greater anisotropy is achieved by hot working the magnetic body. Finally, according to U.S. Pat. No. 4,8842,656 to Maines et al., Lee""s anisotropic magnet body can be crushed to produce a powder suitable for forming anisotropic magnets by powder metallurgy methods.
A process limitation to the use of the above-described permanent magnet compositions is that heating above 1620xc2x0 F. (about 882xc2x0 C.) causes crystallization of the amorphous microstructure, resulting in the loss of the desired magnetic properties. As a result, in achieving a rotating electromagnetic component having both soft and permanent magnet core components, magnet bodies of only relatively weak mechanical strength have been produced; since the sintering (2050xc2x0 F.) required for optimum strength of the soft core component is not possible without destroying the desired anisotropy of the hard core component. Consequently, in the past, rotating electromagnetic components having soft and permanent magnet core components have not been formed exclusively by powder metallurgy techniques, but instead have typically been formed by such methods as placing inserts of permanent magnet material in a sheet lamination stack of a soft magnet material. While capable of achieving desirable magnetic and mechanical properties, such components are costly in terms of materials and assembly.
In view of the above, it would be desirable if a powder metallurgy process were available that enabled the mass production of rotating electromagnetic components having both soft and permanent magnet core components, and which exhibit adequate mechanical strength for applications that require relatively high rotational speeds, as is often the case with AC and DC rotors of motors and generators.
The present invention is directed to a method for manufacturing a rotating electromagnetic component to have both soft and hard (permanent) magnet regions, and in which powder technologies are used to net-shape mold the component. The invention employs a powder of a soft magnet material and either a powder or insert of a hard magnet material, and then performs a partial sintering operation that has been determined to effectively bind the powder (or powders) together without degrading the magnetic properties of the hard magnet material.
The invention generally entails the steps of compacting a soft magnet powder material and an insert or powder of a permanent magnet material to form a rotating electromagnetic body containing soft and hard magnet regions. The partial sintering operation is then performed on the rotating electromagnetic body at a temperature of 1600xc2x0 F. (about 870xc2x0 C.) or less, preferably about 1400xc2x0 F. to 1500xc2x0 F. (about 760xc2x0 C. to about 830xc2x0 C.), and most preferably at 1500xc2x0 F. or slightly below to at least partially fuse the soft magnet powder materials with the permanent magnet material. According to this invention, the soft powder component of the resulting electromagnetic body is sufficiently fused to exhibit mechanical properties comparable to a fully sintered body (i.e., sintered at 2050xc2x0 F. (about 1120xc2x0 C.) or more), but without degrading the magnetic properties of the hard magnet region. The result is a process that enables the mass production of rotating electromagnetic components having both soft and permanent magnet core components, and which exhibit adequate mechanical strength for applications that require relatively high rotational speeds, as is often the case with AC and DC rotors of motors and generators.
Other objects and advantages of this invention will be better appreciated from the following detailed description.